1. Field of the Invention
This invention relates to the field of optical imaging to which it has general application and particularly is related to optometry and the diagnostic equipment used therein, and more specifically ophthalmic imaging equipment and methods for examining the eye and other suitable parts of the body.
2. Description of the Related Art
Visualizing or photographing interior structures of the human eye (i.e., the ocular fundus) is difficult because the entrance pupil of the eye is limited to a diameter of around 10 mm. Through this image area, both illumination pathways and imaging pathways must co-exist but not create glare. Traditional mechanisms to accomplish this function are to provide separate areas of the pupil for an illumination point source, as well as an aerial image of the entrance pupil of the camera through this shared human pupil. Further, interior eye structures must be imaged through the cornea, which inserts in the optical path a highly reflective surface. Thus, through this image area, both illumination pathways and imaging pathways must co-exist and not create glare or undesirable reflections.
A conventional fundus camera as shown in FIG. 1A includes a light source 2 and an optical arrangement for directing light from the light source through the cornea and pupil to the internal eye structure to be imaged. The light from the light source is focused into the eye by an objective lens 4. Light reflected from the eye (e.g., from the retina) travels out of the eye and enters the coupling (i.e., the fundus camera imaging) lens 6, which focuses the image to be captured on an image recorder 8 (i.e., a film or a digital sensor). The light source 2 can be an illumination bulb and/or an electronic flash directed toward the eye by a complex arrangement of optical components that include beam splitters, mirrors, and condensing lenses. As shown in FIG. 1A, in one conventional approach, light from the bulb or flash is reflected from an annular light reflector 10 to form an annulus of light, which is projected into the eye. Light reflected from the eye structure of interest passes out of the eye and through the center of the annular light reflector 10, and back to the image recorder 8. This approach minimizes undesirable reflections by insuring that illumination and reflected light pass through separate regions of the pupil and do not overlap on passing through the cornea. Furthermore, undesirable back reflections from the cornea are minimized in conventional fundus imaging systems by using a black dot in the center of the objective lens 4 to minimize undesirable reflection from the cornea along the central optical axis.
Such fundus cameras in either handheld, mounted, or desktop configurations must accommodate the light source and the associated beam forming and imaging optical elements described above for illumination and imaging, making conventional fundus imaging systems large, heavy, complex, and expensive. While simpler systems for imaging the retina exist, the illumination scheme as shown in FIG. 1B typically use a prism or beam splitter 12 to direct light from the light source 4 into the eye. This approach while simpler than that shown in FIG. 1A suffers due to inadvertent light scattering in the eye. For example, as shown in FIG. 1B, the illumination in this approach enters the eye pupil eccentrically, so the reflection from the cornea does not reflect back into the device, but instead reflects at some angle away from the entrance pupil of the imaging system. Further, the amount of illumination that can be directed into the eye is limited in this approach, and the illumination and imaging pathways overlap. Scattering occurs within the cornea and crystalline lens and obscures the image of the internal structure of the eye.